Tyrannosaurus lurks nearby for their prey to be off guard
A brachiosaurus with the dominant allele for gray-colored skin reproduces with a brachiosaurus with the recessive allele for green-colored skin. They overproduce because life in the cretaceous period was difficult to survive in. They know not all of their offspring will survive because of predators.
The two Brachiosauruses overproduced and made a variation in their offspring since one of them was carrying the recessive gene for green-colored skin. Now there are lots of each color Brachiosaur
#4-Adaptation of Species
A herd of Tyrannosaurs lives nearby and they see the Brachiosaurs. They find that they can see the gray-colored ones much easier than they can the green, so they only hunt down the gray ones.
The green-colored Brachiosaurus thrive
The Tyrannosaurs were more attracted to the gray-colored ones and end up eating those rather than the green ones. The Brachiosaurs develop a structural adaptation that changes the color of their skin over time.
The Brachiosaurs are now all green in color but still carry the dominant gene for gray-colored skin, so it doesn't leave the gene pool forever, but its less common.
The Green Brachiosaurs thrive because they can only see the gray ones, so they move on to hunt other prey instead. This leaves the green Brachiosaurs to keep reproducing.